Tunable cavity resonator



April 17, 1951 E, D, McARTHuR 2,549,499

TUNABLE CAVITY RESONATOR Filed Feb. 19, 1949 Elrwev` D. McArthur,

b5 We fwfy/ His Attorneg.

Patented Apr. 17, 1951 'TUNABLE `oAvI'rY RESONATOR Elmer D. McArthur, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application February 19, 1949, Serial No. 77,366

l My invention relates to cavityresonators of the radial type and has for one of its objects Vthe provision of a radial cavity resonator which is tunable over a wide band of frequencies.

-Radial cavity resonators have heretofore usually been tuned by means of slugs inserted within the resonator itself so as tointercept a preponderance of either magnetic or electrical field lines depending uponl whether it is desired to raise or lower the resonant frequency. These slugs, however, increase the resistive component of the impedance characteristic of the resonator, and if many slugs are used to obtain a substantial frequency adjustment, the efciency of the resonator is thereby appreciablyreduced. Furthermore, due to the consequent distortion of the radial symmetry, these slugs often introduce undesirable higher order Waves within the resonator. Accordingly another object of my invention is to provide a radial cavity resonator in which the natural resonant frequency may be tuned over a fairly wide band of frequencies without the appreciable introductionof additional resistance or higher order waves.

In Vfulliilment of the above objects, it is a further specific object of my invention to provide'` a tuning arrangement for a radial cavity resonator whereby all radii of--the cavity maybe uniformly adjusted simultaneously. I

Broadly stated, my invention comprises a radial cavity resonator wherein the conventional cylindrical wall forming the peripheral enclosure or the resonator is replaced by a plurality of circumferentially displaced resilient `fingers secured to one side of the resonator and making slidable contact with the opposite side thereof. Means are provided to constrict these resilient fingers simultaneously thereby permitting a uniform adjustment of the radial dimension of the resonator.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, together with additional objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which the sole figure is a sectionalview of one embodiment of my invention as employed in conjunction with a centrally positioned ultra high frequency electricl discharge device I, including an anode 2, control grid 3, and a cathode li maintained in consecutively spaced relation along the longitudinal axis of the device by virtue of a pairv of supporting and enclosing cylinders and 6 composed of suitable insulating material such as 2 claims. (c1. 17a-14) glass. A pair of parallel conductive rings 1 and 8 are sealed to opposite ends of insulating cylinder 5 and serve to provide suitable lexternal terminal connections to the anode Zand control grid 3 respectively.

YA pair ofconductive planar members 9 and IU, which may in one form as illustrated be a pair of disks, extend radially from the electric discharge device I and are aligned in parallel fashion to form .opposing sides of a grid-anode cavity resonator. An output coupling connection yfrom the resonator is provided by such means as a coaxial cable I I fastened to planar member 9 and having va coupling probe I I extending within the resonator. The anode connection to the upper planar number 9 is by virtue of a centrally formed depression I2V in which the anode conductive ring 'I is seated. The control grid 3 is preferably capacitively coupled to the' lower planar member I0 in order to permit the application of a uni-directional potential between the anode 2 and the grid 3 and yet provide a coupling which effectively appears as a direct connection in so far as ultra high frequency potentials are concerned. A conductive annular plate I3 separated from the lower planar member Ill by an insulating washer I3 and connected to the grid conductive ring 8 by an annular spring member I4 may be employed to accomplish this capacitive coupling,

The peripheral enclosure of the resonator is formed by a plurality of embowed resilient conductive ngers I5 rigidly secured to circumferentially displaced points on one of the planar members such as lower planar member II] and pressing tightly against the inside conductive surface of the opposing planar member, which inthe instant embodiment is designated as member 9. The embowed configuration and resilient composition of the fingers I5 create a tension therein which exerts a strong pressure against the opposing planar memberg in order to make good contact and provide a low loss ultra high frey quency .connection therewith. This tension with;Y

inthe fingers also causes them to tend to move centrifugally.

Inorder to restrain this centrifugal motion and coincidently to provide one convenient means of adjusting their radial position, I preferably em'- ploy an annular shoe or plunger I6 encircling the resonator and having an annular concave depression fitting contiguously against the convex outer surface of the embowed resilient fingers I5.l This annular shoe I6 is slidably inserted within a pair of telescoped guiding cylinders I1 and I8 and is adjusted to press against the resilient fingers I5 by any suitable mechanical means (not shown). A movement of annular shoe i6 toward planar member 9 constricts these resilient fingers l5 and forces their free ends to slide uniformly toward the center along the surface of planar member 9 thereby decreasing the radial dimensions of the resonator. A movement of the annular shoe IB in the opposite direction permits the centrifugal expansion of these ngers thereby increasing the radial dimensions of the resonator again.

When a cavity resonator of the type herein de-r scribed is excited at its natural vresonant fre-` quency by a proper ultra high frequency electric discharge through device l, a standing wave is produced therein having a changing magnetic field forming closed loops normal to the axis of the discharge device I and a changingY electrical field which is normal to the magnetic field and Whose lines extend between the conductive planar member 9 and I0. This mode of propagation, commonly called a transverse magnetic mode of the rst order, (TMm) results -in a voltage antinode at the central axis and a voltage-node at the periphery of the resonator.

The natural resonate frequency of a cylindrical cavity resonator utilizing a TMm mode of propagation is given by theformula v where c is the velocity of light and a is'the radius of the cavity. As is indicated by the formula, the resonant frequency in this mode is a function of only one dimension, i. e., the radius. The other dimensions are not required to fit any particular number or half or whole periods of variations and hence do not directly affect the resonant frequency.

The cavity resonator herein described is substantially equivalent to a cylindrical cavity resonator, and the above formula may be employed to calculate the approximate resonant frequency thereof. Since the natural resonant frequency of the resonator is dependent upon the radius and since the average'radius` of the resonator is directly varied by the movement of fingers l5, its natural resonant frequency is thereby also varied proportionally.

It will be appreciated that since my invention employs only one set of sliding contacts'instead of the two sets of sliding contacts employed in conventional piston tuned resonators, the contact losses are thereby appreciably reduced. Furthermore, since the irregularity of the periphery of the .resonator caused by the embowed configuration of the resonant fingerslv affects only a relatively small part of the total surface and is completely symmetrical, the efficiency of the resonator is not substantially aiiected,and the possibility of the introduction of higher order waves is minimized.

It will be appreciated that although I have shown' a cavity resonator employed in conjunction with a conventional centrally positioned electric discharge device, neither the particular discharge device employed nor the means by which the cavity resonator is excited form part of my present invention. My invention is contional Klystron oscillator and excited by the.

energy transferred to the resonator from a velocity modulated electron stream.

It is also to be understood that while I have shown one particular embodiment of my invention, many modifications may be made, and I, therefore, contemplate by the appended claims to cover all such modifications as fall Within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A radial cavity resonator comprising a pair of parallel electrically conductive planar members aligned to form opposing sides of said resonator, a plurality of circumferentially spaced outwardly embowed resilient electrically conductive fingers forming a convex periphery of said resonator, each nger being secured to the same one of said members and making firm slidable contact with the opposing planar surface of the other of said members under a centrifugal tensional force provided by its outwardly embowed configuration, and means encircling said fingers to constrict said fingers radially and uniformly to move the points of contact of said fingers on said opposing surface against the centrifugal tensional force of said fingers to adjust the natural resonant frequency of said resonator.

2. In combination, aA radial cavity resonator comprising a pair of parallel electrically conductive planar members aligned to form opposing sides thereof, and a plurality of outwardly embowed resilient electrically conductive fingers' forming a convex periphery of said resonator, said fingers being secured at one end to circumferentially spaced points on one of said members and making firm slidable contact at the other end with the opposing surface of the other ofA said members under a centrifugal tensional force provided by their outwardly embowed configuration, an annular shoe encircling said fingers and having a concave internal surface fitting contiguously against the embowed outer surfaces of said fingers, and means to vary the position of said annular shoe relative to said fingers in a direction parallel to the axis of said radial resonator to adjust the radial points of contact ofj said fingers on said other planar member, by constricting said fingers radially and uniformally against the tensional force thereof, thereby to tune said resonator.

ELMER D. McARTHUR.

Name Date Hansen Aug. 27, 1946 Number 

